This invention relates to an improvement in disposable liquid dispensing apparatus, and more particularly to such an apparatus for the intravenous or enteric infusion of accurately measured amounts of liquid, such as medicaments or nutrients, to a patient.
When intravenously or enterically infusing liquids into a patient, it is often desirable and sometimes absolutely necessary to control the specific amount of fluid which is to be administered, as well as the rate of administration, so that the accurate control of dosage is possible. It is conventional in such dispensing apparatus to provide a separate drip or drop counting chamber in the liquid conveying conduit between a liquid supply reservoir and the patient to provide a visual flow indication of the liquid being administered. The determination of the dose is usually made by providing some drop forming means at the upper end of the drop count chamber, and counting the drops falling to the bottom of the drip chamber from which the liquid is conveyed to the patient.
Conventionally, the dosage is determined by calibration of the drop count per unit time multiplied by the volume of liquid contained in each drop. The drop size is determined by the physical configuration of the drop forming means and is usually fixed for each drop forming means, and the drop rate is conventionally controlled by a pinch type device which clamps or restricts the flow through the liquid conveying conduit.
Since the entire liquid dispensing apparatus, also referred to as a fluid administration set or an IV (intravenous) set, has to retain sterile integrity, they are generally of the disposable or throw away after use type. This requires it to be inexpensive to manufacture, and since accurate control of the dosage requires drops of the same volume from IV set to IV set, the drop forming means must be the same for each IV set.
The conventional drop forming means for a disposable IV set is a molded plastic part, such as the stopper illustrated in U.S. Pat. No. 3,311,268, which closes the upper end of the drop counting chamber and which has a passage therethrough whose diameter is preselected to dispense drops of a certain size. It is well known to those skilled in the art that the size of the drops released from such a passage is usually a function of the diameter of the passage. Such drop forming means-stoppers are generally formed by a female injection molding die utilizing a slightly tapered male injection molding die to form the drop forming liquid passage. One of the problems encountered in the mass production of such drop forming means is that dies wear out because of sliding friction between the male part of the die and the molded piece, and after the molding of several hundred thousands of prior art drop formers, the constant wear on the tapered male injection molding die makes the same smaller with use and thereby reduces the diameter of the drop forming passage, and thereby the volume of the drops it forms.
Since the disposable drop chamber is generally inserted into a permanent metering means which determines the drop rate and actuates a drop rate control means to adjust the drop rate for the desired dosage, it becomes of utmost importance that the volume of the drops formed remains constant from drop former to drop former, else it becomes necessary to calibrate each disposable liquid administration set prior to utilization.
U.S. Pat. No. 3,276,639 discloses one form of a drop former for dispensing drops of liquid of uniform and accurately predetermined volume. That patent proposes to provide a special drop forming element at the end of a drop forming passage in the form of a tube which has a flared discharge opening. While this extension may accomplish the stated objects and provide drops of a desired volume, if it is also an injection molded part, the dies producing this part will wear after many uses causing a slight and continuous change in the dimension of the parts whereby this particular drop former suffers from the same deficiency, namely the inability of each manufactured drop former to deliver drops of the same precise and uniform volume as the manufacturing dies for producing these extensions start to wear.